Measuring the Gluon Density in e + A Collisions: KLN CGC, DGLAP Glauber, or Neither?

TL;DR

This paper compares predictions of nuclear gluon distributions in e+A collisions using DGLAP Glauber and KLN CGC models, highlighting significant differences in diffraction patterns and cross section behaviors at various energies.

Contribution

It provides a detailed comparison of DGLAP Glauber and KLN CGC models for nuclear gluon distributions, predicting observable differences in diffraction and cross sections in e+A collisions.

Findings

CGC predicts a widening of the gluon density with x, affecting diffraction patterns.

DGLAP Glauber yields x-independent diffraction peaks and minima.

Dipole cross section with CGC reaches the black disk limit, unlike DGLAP.

Abstract

We predict readily experimentally measurable differences in the diffractive cross section in the coherent exclusive photoproduction of J/psi mesons in e + A collisions at eRHIC and LHeC energies for nuclear gluon distributions assumed to 1) evolve in x with DGLAP dynamics and have a spatial distribution proportional to the Glauber nuclear thickness function and 2) evolve in x and b according to the KLN prescription of CGC dynamics. We find that CGC physics predicts that the nuclear gluon density widens significantly as a function of x yielding diffractive peaks and minima that evolve dramatically with x; on the other hand the DGLAP Glauber distribution yields peaks and minima constant in x. We also find that the dipole cross section at the level of two gluon exchange within the KLN parameterization of the CGC satisfies the black disk limit whereas this limit is violated when DGLAP evolution is used; the normalization of the diffractive cross section grows more slowly in x by several orders of magnitude when using the KLN parameterization as compared to the result when employing DGLAP evolution.